Baker's dozen. The date today. The number of original American colonies.
If, after all that, you're feeling a little breathless, starting to sweat and your heart is palpitating, it may be triskaidekaphobiathe irrational fear of the number 13. In extreme cases, triskaidekaphobia can bring on an anxiety attack, which may seem completely ridiculous to someone who does not suffer from the condition (in fact, those same people may find it laughable that there's even a name for this "condition"). But consider this: Napoleon, Herbert Hoover, Mark Twain and Franklin Roosevelt all considered themselves triskaidekaphobes.
For Christians, 13 is considered to be unlucky because it was the number of people present at the Last Supper. Friday is also considered to be unlucky in the Christian church because Jesus was crucified on Friday, therefore a Friday that also falls on the 13th day of the month is considered especially unlucky. A recent theory offered forth by Umberto Eco in his novel Foucault's Pendulum is that Friday the 13th is unlucky because it is said to be the day that the Knights of the Templar, the powerful Christian army of the Middle Ages, were decimated by the king of France.
Pop culture has played heavily on this old Friday the 13th series. And in many buildings with at least 13 stories, the 13th floor has been omitted, with the floors instead being numbered sequentially from one to 12 and then 14 and higher.
Just for the record, triskaidekaphobia may be the irrational fear of the number 13, but those with an irrationaland often completely uncontrollablefear of Friday the 13th suffer from a different condition, paraskavedekatriaphobia.